The present invention relates to a scanning projection exposure apparatus for exposing, in a process for manufacturing a semiconductor integrated circuit or liquid display element, the pattern of a mask (photomask or reticle) as a master onto a substrate (wafer or glass plate) and, more particularly, to a scanning projection exposure apparatus for aligning the master and substrate at high precision when deformation such as deflection occurs in the master.
In a photolithography process for manufacturing a semiconductor integrated circuit or the like, a projection exposure apparatus is used, which exposes a pattern image of a master (photomask or reticle) onto a substrate (wafer or glass plate) coated with a photoresist or the like. In this projection exposure apparatus, as the feature size of the circuit pattern as the transfer of the target decreases, the allowable range of the fluctuation amount of the image-forming characteristics of a projection image formed by a projection optical system narrows. To solve this problem, conventionally, in order to correct the fluctuation amount of the image-forming characteristics (e.g., magnification, focal position, and the like) occurring upon absorption of illumination light with a projection optical system, the projection exposure apparatus has an image-forming characteristic correcting mechanism, as disclosed in Japanese Patent Laid-Open No. 60-78455 or No. 63-58349, which detects the quantity of light incident on the projection optical system and corrects the fluctuation amount of the image-forming characteristics of the projection optical system in accordance with the detected quantity of light.
For example, a mechanism disclosed in Japanese Patent Laid-Open No. 60-78455 will be briefly described. A model corresponding to the fluctuation characteristics of the image-forming characteristics of the projection optical system is formed in advance. The quantity of light energy which becomes incident on the projection optical system at a predetermined time interval is detected by a photosensor or the like on a wafer stage upon which a wafer as the substrate is placed. The integral value of the quantity of light energy is applied to this model, and the fluctuation amount of the image-forming characteristics is calculated. In this case, an exposure time for which the integral value of the light energy incident on the projection optical system is to be obtained is calculated by, e.g., constantly monitoring a signal indicating that a shutter for opening/closing illumination light is in the open state. Hence, the current fluctuation amount of the image-forming characteristics of the projection optical system can be calculated in accordance with this model, and correction is performed on the basis of the fluctuation amount. For the time being, this solves the problem of fluctuation of the image-forming characteristics, which is caused upon absorption of the illumination light with the projection optical system.
As the illumination light also passed through a mask serving as a master, the mask thermally deforms upon absorption of the illumination light, and consequently, the image-forming characteristics are changed. Particularly, since a pattern is drawn on the mask with a light-shielding film such as a chromium film, heat absorption of the light shielding film is large, unlike in a glass substrate portion with a high transmittance. In recent years, for the purpose of preventing flare of the optical system, a technique that decreases reflection of the light-shielding film on the mask as been introduced. This further increases heat absorption with the light-shielding film.
A circuit pattern formed with the light-shielding film on the mask is not always distributed uniformly over the entire mask, but may sometimes be distributed nonuniformly. In this case, the temperature of the mask increases locally to likely cause anisotropic distortion. When a variable field stop (reticle blind) or the like is used to expose the pattern on the mask only partially, anisotropic distortion similarly occurs. This distortion in the mask leads to anisotropic distortion in the projected image.
Concerning the thermal deformation of the mask, since the thermal deformation amount, and moreover, the change amount of the image-forming characteristics, change depending on the type of the mask employed, they are difficult to correct uniformly. In other words, the amount of fluctuation, occurring upon thermal deformation, of the image-forming characteristics of a mask used for adjusting the image-forming characteristics of a projection exposure apparatus before shipping may be recognized as the fluctuation characteristics of the image-forming characteristics of this projection exposure apparatus, and may be corrected accordingly. When another mask is used, its thermal deformation amount differs, and accurate correction cannot be performed. Particularly, when exposure is to be performed by successively changing masks, unless the thermal deformation amounts of the respective masks are considered, the fluctuation amounts of the image-forming characteristics are accumulated to likely cause a large error.
As a countermeasure for this, Japanese Patent Laid-Open No. 4-192317 discloses a projection exposure apparatus which corrects a change in optical characteristics that occurs due to thermal deformation of a mask, while including the heat absorption ratio of chromium which makes up the mask pattern and the content of chromium in the pattern in the parameters.
In this prior art, correction is performed merely based on calculation, and many errors exist with respect to the actual expansion amount. For example, while heat absorbed by the mask is diffused into air by radiation and convection, it is very difficult to describe this phenomenon with a mathematical expression precisely. However, unless heat absorbed by the mask and heat emitted from the mask are estimated accurately, the expansion amount of the mask cannot be calculated.
In recent years, a projection exposure apparatus employing a so-called step-and-scan exposure method or slit-scan exposure method (to be referred to as a xe2x80x9cscan exposure methodxe2x80x9d hereinafter) has been developed, which illuminates a mask pattern region in a slit manner, scans a mask with respect to the slit-like illumination region, and scans a wafer with respect to an exposure area conjugate to the slit-like illumination region in synchronism with scanning of the mask, thereby sequentially projecting and exposing the pattern of the mask onto the respective shot regions of the wafer. According to this scan exposure method (scanning type), a large area can be exposed without being limited by the field size of the projection optical system in the scanning direction.
In this scan exposure method, during exposure, the mask is scanned with respect to the illumination region. Accordingly, factors that should be considered regarding the mask (e.g., the cooling effect of the mask accompanying mask scanning) increase, and calculation of the thermal deformation amount of the mask becomes more complicated than in cell projection exposure. Considering the foregoing, to cope with deformation of the mask, it is more effective to measure the deformation amount of the mask directly rather than to obtain the deformation amount by calculation as in the prior art.
As prior art in consideration of this aspect, Japanese Patent Laid-Open No. 4-192317 discloses a method of measuring the deformation amount of a mask. According to the method disclosed in this application, the temperature distribution of a reticle is detected by a non-contact temperature sensor such as an infrared camera, thereby obtaining the deformation amount, or a mark is formed in the periphery of a reticle, and a displacement of this mark is detected by a detection system arranged above the reticle, thereby obtaining deformation. According to this method, in detection of the mark on the reticle with the detection system, if the deformation amount of the mask is to be obtained from the position of the mask relative to the position of a reference mark formed on a wafer stage, the position of the wafer stage is limited during measurement. During mask measurement for obtaining deformation of the reticle, the wafer stage must be controlled such that the reference mark on it is at a predetermined position, and operation such as wafer exchange cannot be performed. This decreases the throughput greatly. As a countermeasure for this, the reference mark may not be used, but the mark on the reticle can be detected with reference to the detection system without using the reference mark. Then, however, the driving precision of the detection system adversely affects the measurement precision, and detection cannot be performed at high precision.
Japanese Patent Laid-Open No. 10-64811 discloses a method of measuring deformation of the mask by forming a reference mark near the mask. According to this method, a reference mark is formed in a projection optical system on the mark side, and the deformation amount of the mask is obtained from the position of the mark on the mask relative to the position of the reference mark with a detection means set above the mask. It is, however, difficult to set the mask and projection optical system close to each other, and at least a mask holding mechanism must be arranged between the mask and projection optical system. Therefore, the mark on the mask and the reference mark on the projection optical system are set separate from each other by approximating several tens of mm. To observe the two marks simultaneously with the detection means, the detection means must have a depth of focus which is equal to the distance or more between the two marks, and it is optically difficult to set such a detection means. When the two marks are to be measured separately, the focal point of the detection system must coincide with the respective marks in each measurement. To implement this, the detection system must have a mechanism which performs focus adjustment by moving a lens group in the direction of an optical axis. A driving error (shift or inclination due to pitching, yawing, and the like, during driving) of the adjusting mechanism appears as movement of an observation image on the sensing surface, which adversely affects the measurement precision.
Japanese Patent Laid-Open No. 10-64811 also discloses a method of providing four reference marks on a mask stage in a scan exposure apparatus and measuring a mark on a mask and the reference marks with two detection means set above the mask. According to this method, the shifts of the two marks from the corresponding reference marks are measured by the detection means. Then, the mask stage is moved in the scanning direction, and shifts of the two remaining marks from the corresponding reference marks are measured.
Inconveniences of this method will be described. In the scan exposure method, to increase the throughput of the apparatus, the scanning speed must be increased. To increase the scanning speed of a photosensitive substrate, the scanning speed of the mask stage must also be increased. The scanning speed of the mask speed must be xcex2 times the scanning speed of the photosensitive substrate, where xcex2 is the magnification of a projection optical system. To satisfy this speed requirement while decreasing the scanning stroke to the necessary minimum, the acceleration of the stage must be increased. If the acceleration of the stage is increased, the holding force with which the mask is held by the mask stage must be large so it can endure the acceleration. Generally, since a mask is held by suction, a large suction area must be reserved. To realize this method, however, a deformation amount measurement mark must be arranged around the mask, i.e., in the mask chucking portion. This is contradictory to the increase in suction force. To improve the measurement precision, a larger number of marks must be arranged on the mask and the mask stage. To increase the throughput, the suction area must be increased in order to increase the mask holding force. This is contradictory to ensuring a space where a larger number of marks is to be arranged.
In the scan exposure method, a detection system for mask alignment is sometimes provided at a mask transfer position. This will be described with reference to FIG. 3. Referring to FIG. 3, a mask stage RST has moved to a transfer position for a mask R. A mask alignment detection system 24 is set above the mark R. A detection system 23, which functions as a TTL detection system for a wafer is set above a projection optical system 8. A reference mark 21 for aligning the mask R is arranged on the mask stage RST. Other arrangements will be described later and a description thereof will accordingly be omitted here. In the above arrangement, to detect the deformation amount of the mask R, it can be measured with reference to the reference mark 21 by the mask alignment detection system 24. However, according to this method, since measurements are performed at two, right and left positions, highly precise measurements cannot be performed. If the mask alignment detection system 24 is provided with a driving system in a direction perpendicular to the scanning direction in order to perform measurements at a plurality of points, the driving precision adversely affects the measuring precision, and highly precise measurements cannot be performed. To perform measurements at a plurality of points in the scanning direction, the suction area described above must be ensured. Also, the stroke of the mask stage RST is not sufficient up to the transfer position of the mask R, and the stroke must be increased by an amount corresponding to a stroke necessary for mark measurement. Unless the stroke of the mask stage RST further extends in a direction opposite to the projection optical system 8 from the transfer position of the mask R shown in FIG. 3, the mark arranged between the transfer position and the projection optical system 8 cannot be measured. If the stroke of the mask stage RST is increased to enable measurement, the entire apparatus size increases and the cost increases.
It is an object of the present invention to align, particularly in a scanning projection exposure apparatus, a mask as a first object and an exposure target substrate as a second object at high precision, and for this purpose to provide means for measuring and correcting the deformation amount, particularly, thermal expansion amount, of the mask. It is another object of the present invention to perform measurement of the deformation amount, particularly, thermal expansion amount, without adversely affecting the throughput of a scanning projection exposure apparatus, and to provide means for performing measurement at high precision with a mask alignment detection system and a TTL detection system usually formed in a scanning exposure apparatus, without increasing a stroke of a mask stage.
In order to achieve the above objects, according to the present invention, there is provided a scanning projection exposure apparatus, which, as a first movable stage which moves with a first object being placed thereon, and a second movable stage which moves with a second object being placed thereon, scans the first and second movable stages in synchronism with each other with respect to a projection optical system, and projects a pattern formed on the first object onto the second object through the projection optical system, characterized by comprising a reference plate fixed to the first movable stage and having a mark, and a detection system for detecting a position of the mark formed on the reference plate and a position of a mark formed on the first object, wherein a deformation amount of the first object is obtained on the basis of the position of the mark formed on the reference plate and the position of the mark formed on the first object, which are detected by the detection system.
According to the present invention, there is also provided a scanning projection exposure apparatus which has a first movable stage which moves with a first object being placed thereon, and a second movable stage which moves with a second object being placed thereon, scans the first and second movable stages in synchronism with each other with respect to a projection optical system, and projects a pattern formed on the first object onto the second object through the projection optical system, characterized by comprising a reference plate fixed to the first movable stage and having a mark, a first detection system for detecting a position of the mark formed on the reference plate, and a second detection system for detecting a position of a mark formed on the first object, wherein a deformation amount of the first object is obtained on the basis of the position of the mark formed on the reference plate and detected by the first detection system, and the position of the mark formed on the first object and detected by the second detection system.
According to the present invention, there is also provided an aligning method for a scanning projection exposure apparatus, which scans a first movable stage which moves with a first object being placed thereon, and a second movable stage which moves with a second object being placed thereon, in synchronism with each other with respect to a projection optical system, and projects a pattern formed on the first object onto the second object through the projection optical system, characterized by comprising the steps of positioning the first object with respect to the first movable stage, and detecting a position of a mark formed on a reference plate fixed to the first movable stage by a detection system and obtaining a detection reference position of the detection system from a position detection result, and detecting a position of a mark formed on the first object with the detection system, thereby obtaining a deformation amount of the first object.